________________________________________________________________Wave Propagation
cause a low-pitched sound. Thus, the frequency of the wave determines pitch.
When the frequency is low, sound waves are long; when it is high, the waves
are short. A sound can be so high in frequency that the waves reaching the
ear cannot be heard. Likewise, some frequencies are so low that the eardrums
do not convert them into sound. The range of sound that the human ear can
detect varies with each individual.
Sound Intensity
1-62. The intensity of sound, at a given distance, depends upon the
amplitude of the waves. Thus, a tuning fork gives out more energy in the
form of sound when struck hard than when struck gently. You should
remember that when a tuning fork is struck, the sound is omnidirectional
(heard in all directions), because the sound waves spread out in all directions,
as shown in figure 1-14. You can see from the figure that as the distance
between the waves and the sound source increases, the energy in each wave
spreads over a greater area; hence, the intensity of the sound decreases. The
speaking tubes sometimes used aboard a ship prevent the sound waves from
spreading in all directions by concentrating them in one desired direction
(unidirectional), producing greater intensity. Therefore, the sound is heard
almost at its original intensity at the opposite end of the speaking tube. The
unidirectional megaphone and the directional loudspeaker also prevent sound
waves from spreading in all directions.
Figure 1-14. Sound Waves Spread in All Directions
1-63. Sound intensity and loudness are often mistakenly interpreted as
having the same meaning. Although they are related, they are not the same.
Sound intensity is a measure of the sound energy of a wave. Loudness, on the
other hand, is the sensation the intensity (and sometimes frequency) the
sound wave produces on the ear. Increasing the intensity causes an increase
in loudness but not in a direct proportion. For instance, doubling the loudness
of a sound requires about a tenfold increase in the intensity of the sound.
1-19
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